A couple of weeks ago Dragisa Krsmanovic asked a question here about how to use the free monad in Scalaz 7 to avoid stack overflows in this situation (I've adapted his code a bit):
import scalaz._, Scalaz._
def setS(i: Int): State[List[Int], Unit]…
In the past 2 years, I was interested in using free monad to helping me to solve practical software engineering problem. And came up my own construction of free monad using some elementary category theory.
{-# LANGUAGE RankNTypes #-}
import…
In blog posts and examples by Mark Seemann, I got a first glimpse of free monads as a way to structure the boundary between pure code and IO code. My basic understanding is that a free monad lets you build a program (an abstract syntax tree - AST)…
In this article about the Free Monads in Haskell we are given a Toy datatype defined by:
data Toy b next =
Output b next
| Bell next
| Done
Fix is defined as follows:
data Fix f = Fix (f (Fix f))
Which allows to nest Toy expressions by…
I've been using the Free datatype in Control.Monad.Free from the free package. Now I'm trying to convert it to use F in Control.Monad.Free.Church but can't figure out how to map the functions.
For example, a simple pattern matching function using…
Ok, so I have figured out how to implement Reader (and ReaderT, not shown) using the operational package:
{-# LANGUAGE GADTs, ScopedTypeVariables #-}
import Control.Monad.Operational
data ReaderI r a where
Ask :: ReaderI r r
type Reader r a =…
On page 12 of One Monad to Prove Them All, it is written that "a prominent example [of container] is the list data type. A list can be represented by the length of the list and a function mapping positions within the list".
At first I thought that…
I'm trying to work with an indexed free monad (Oleg Kiselyov has an intro). I also want that free monad to be built from a from a coproduct of functors a la Data Types a la carte. However, I'm having trouble getting the coproduct injection type…
Most monadic functions take pure arguments and return a monadic value. But there are a few that need also monadic arguments, for example:
mplus :: (MonadPlus m) => m a -> m a -> m a
finally :: IO a -> IO b -> IO a
forkIO :: m () -> m ThreadId
--…
I'm attempting to define an abstract syntax type using ekmett's libraries bound and free. I have something working, which I can strip down to the following minimal example:
{-# LANGUAGE DeriveFunctor #-}
import Bound
import Control.Monad.Free
type…
I'm having a bit of a hard time understanding how to prove the Functor and Monad laws for free monads. First off, let me put up the definitions I'm using:
data Free f a = Pure a | Free (f (Free f a))
instance Functor f => Functor (Free f) where
…
I am trying to abstract the pattern of applying a certain semantics to a free monad over some functor. The running example I am using to motivate this is applying updates to an entity in a game. So I import a few libraries and define a few example…
So we have the free monad: (encoding may vary, but they're all the same)
data Free f a = Pure a
| Impure (f (Free f a))
instance Functor f => Monad (Free f) where
pure = Pure
Pure x >>= f = f x
Impure x >>= f = Impure…
So I understand that a free object is defined as being the left-hand side of an adjunction. But how does that lead you to the Haskell definition of such objects?
More concretely: given a "forgetful functor" from the category of monads to the…
This question is related to this article
The idea is to define a DSL for manipulating files in the cloud, and define a
composition of interpreters that take care of the different aspects, such as
communication with the REST interface and…
